g-C3N4 with enhanced visible light photocatalytic performance

Accepted Manuscript Title: Self-assembly synthesis of BiVO4 /Polydopamine/g-C3 N4 with enhanced visible light photocatalytic performance Authors: Rui Huo, Xue-Ling Yang, Jing-Yu Yang, Si-Yuan Yang, Yue-Hua Xu PII: DOI: Reference:

S0025-5408(17)32807-6 https://doi.org/10.1016/j.materresbull.2017.10.016 MRB 9623

To appear in:

MRB

Received date: Revised date: Accepted date:

19-7-2017 12-9-2017 10-10-2017

Please cite this article as: Rui Huo, Xue-Ling Yang, Jing-Yu Yang, SiYuan Yang, Yue-Hua Xu, Self-assembly synthesis of BiVO4/Polydopamine/gC3N4 with enhanced visible light photocatalytic performance, Materials Research Bulletin https://doi.org/10.1016/j.materresbull.2017.10.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Self-assembly synthesis of BiVO4/Polydopamine/g-C3N4 with enhanced visible light photocatalytic performance Rui Huoa, Xue-Ling Yangb, Jing-Yu Yanga, Si-Yuan Yanga, Yue-Hua Xua,* a

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China b

Guangzhou CAS Test Technical Services Co., Ltd., Guangzhou 510650, China

*Corresponding Author E-mail address: [email protected] (Yue-Hua Xu) Graphical abstract

Highlights 

BiVO4/PDA/g-C3N4 was synthesized via simple self-assembly at the room temperature.



PDA has a strong binding affinity to BiVO4 and g-C3N4 to form BiVO4/PDA/g-C3N4.



BiVO4/PDA/g-C3N4 exhibits good stability and reusability in the photocatalytic reaction of glyphosate.



The enhanced activity of BiVO4/PDA/g-C3N4 is mainly attributed to the increased 1

O

charge separation.

ABSTRACT: BiVO4/PDA(Polydopamine)/g-C3N4 was synthesized by facile ultrasonic dispersion and self-assembly at the room temperature, and monoclinic BiVO4 was embedded on PDA/g-C3N4 sheet, which PDA was first formed in situ on the surface of g-C3N4 via self-polymerization of dopamine. The dependence of photocatalytic activity on the structural, electronic and optical properties of the BiVO4/PDA/g-C3N4 nanostructure has been explored using XRD, XPS, TEM, SEM, DRS, PL, BET and photocurrent. The BiVO4/PDA/g-C3N4 photocatalysts show higher activity of glyphosate degradation than BiVO4, g-C3N4 and BiVO4/g-C3N4 under visible light irradiation. The enhanced activity of BiVO4/PDA/g-C3N4 can be significantly attributed to the improved visible light absorption, enhanced specific surface area and increased charge separation. PDA has a strong and irreversible binding ability onto BiVO4 and g-C3N4 to form BiVO4/PDA/g-C3N4. This simple method at the room temperature is promising for the synthesis of high performance composite photocatalysts for different applications, such as pollutant degradation, photocatalytic splitting water, etc.

Keywords: BiVO4, g-C3N4, Polydopamine, Glyphosate, Photocatalytic 1. Introduction Semiconductor photocatalysis has emerged as a promising technology for pollutant removal from water. TiO2 is the most popular photocatalyst for the pollutant degradation reported in previous studies due to its high activity, low cost and non-toxicity [1-3]. However, TiO2 requires the use of UV light. Therefore, more recent efforts have been aimed to develop efficient visible-light-active photocatalysts, which can utilize the largest fraction of the solar spectrum, 2

for the mineralization of organic pollutants. Among the many explored photocatalysts, bismuth vanadate (BiVO4) is one of the most promising photocatalytic materials for organic pollution degradation due to its narrow band gap (2.4 eV), low cost and high stability [4-6]. Compared to a single component semiconductor, the construction of heterojunction structure between two semiconductors exhibits superior performance caused by the enhanced separation efficiency of the photogenerated electron−hole pairs [7]. For instance, CeO2/BiVO4 [8], In2O3/BiVO4 [5], SiC/BiVO4 [9], and NiO/BiVO4 [10] heterojunction composites have shown higher photocatalytic activity than BiVO4 alone. These heterostructures can be tailored by fine-tuning the composition, size and shape of two semiconductors, which can improve the charge transfer and subsequently enhance the separation of photogenerated carriers. Graphite carbon nitride (g-C3N4), a polymeric metal-free photocatalyst, has attracted considerable attention because of its chemical and thermal stability, large surface area and narrow band gap (Eg = 2.7 eV) [11, 12]. In addition, g-C3N4 is commercially available and also can be prepared simply by pyrolysis or polycondensation of nitrogen-rich organic precursors such as urea, cyanamide, thiourea, melamine and other s-triazine derivatives. The conductance band of g-C3N4 (ECB= 1.13 eV vs. NHE) [13] is more negative than that of BiVO4 (ECB= 0.35 eV) [14], while the valence band (VB) of BiVO4 (EVB = 2.75 eV) is more positive than that of g-C3N4 (EVB = 1.57 eV). Therefore, g-C3N4 and BiVO4 possess matchable band energy levels for constructing an effective heterojunction, which can enable the efficient transfer and separation of photogenerated charge carriers in the BiVO4/g-C3N4 composite. The BiVO4/g-C3N4 photocatalysts with enhanced photocatalytic activities have been prepared using different methods, such as powder mixing-calcination method [15], ultrasonic dispersion method [16], and ultrasonic dispersion and calcination method [17, 18], hydrothermal-chemisorption method [19]. Dopamine (DA) can facilely self-polymerize to form a controlled thin, mussel-inspired and 3

stable polydopamine (PDA) layer on all inorganic and organic materials‫ ׳‬surfaces [20]. The amino and catechol groups in the PDA have a strong and irreversible binding ability onto inorganic and organic materials [20, 21]. In this study, the BiVO4/PDA/g-C3N4 composite photocatalysts with enhanced photocatalytic activity were synthesized via facile ultrasonic dispersion and DA self-assembly at room temperature. We report a systematic study on a strategy combined polymerization of PDA/g-C3N4, which the PDA coating was formed in situ on the surface of g-C3N4 through self-polymerization of DA, with BiVO4 deposition to simplify the synthetic steps and conditions. Here the BiVO4/PDA/g-C3N4 nanostructures were explored the dependence of the enhanced photocatalytic performance of glyphosate degradation under visible light irradiation on their structural, electronic and optical properties.

2. Experimental section 2.1 Synthesis of BiVO4 and g-C3N4 BiVO4 was prepared by the chemical precipitation method as follows [22]: 8.985 g of Bi(NO3)3·5H2O was completely dissolved in 150 ml of 1.34 M CH3COOH by ultrasonication to form solution A. 2.166 g of NH4VO3 was dissolved in 150 ml of 0.5 M NaOH by ultrasonication to form solution B. Subsequently, the solution B was added to the solution A by ultrasonication to form a yellow mixture suspension, and the BiVO4 precursor was separated by filtration. The BiVO4 precursor was washed with water and ethanol, and was dried at 70 oC. Finally, the BiVO4 precursor was calcined at 400 oC for 2 h. The yellow g-C3N4 powder was fabricated by heating urea powder at 550 oC for 4 h. In a typical experiment, 100 mg of g-C3N4 was dispersed in 100 ml of alcohol by ultrasonication for 2 h, and then a PDA/g-C3N4 suspension was prepared via self-assembly after adding 20 mg of dopamine hydrochloride by stirring at room temperature for 12 h for further use. The g-C3N4 sample was also prepared using the same method, but no dopamine hydrochloride was added. 2.2 Synthesis of BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 4

In a typical preparation procedure, 0.170 g of BiVO4 was dispersed in 50 ml of alcohol by ultrasonication for 2 h. Subsequently, 30 mL of PDA/g-C3N4 suspension was added to this solution followed by stirring at room temperature for 12 h. Finally, the resulting suspension was dried by stirring at 60 oC to evaporate alcohol, and the BiVO4/PDA/g-C3N4 sample was fabricated via self-assembly. For comparison purposes, the BiVO4/g-C3N4 sample was prepared using the same method, but a g-C3N4 suspension substituted the PDA/g-C3N4 suspension. 2.3 Characterization The X-ray diffraction (XRD) patterns of the as-prepared photocatalysts were carried out using a MSALXD-2 X-ray diffractometer with Cu Kα radiation (λ=0.15406 nm). The chemical composition of the as-prepared photocatalyst surface was determined by X–ray photoelectron spectroscopy spectra (XPS). XPS analysis was performed on an ESCALAB 250 high performance photoelectron spectrometer (Thermo Fisher Scientific) with pass energy of 20 eV using Al Kα radiation. The binding energy values were all calibrated using the contaminant C 1s peak at 284.8 eV. The surface morphologies of the samples were studied using a JSM- 6700F scanning electron microscopy (SEM) and a FEI-Tecnai 12 transmission electron microscopy (TEM). The absorption properties and band gaps of the powders were examined using a HITACHI U-2550 UV-vis spectrophotometer over a wavelength region of 300−800 nm, and BaSO4 was used as the reference material. The photoluminescence (PL) spectra of the samples were measured using Shimadzu RF-5301PC spectrophotofluorometer with an excitation wavelength of 278 nm. The photocurrent response was measured using an electrochemical station (Epsilon-BAS) in a standard three-electrode system. A 300 W Xe lamp with a UV cut-off filter (λ≥420nm) was used as a visible light source, and a 0.5 M Na2SO4 solution was used as the electrolyte solution. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate was used as the counter electrode. The working electrodes were BiVO4, g-C3N4, BiVO4/g-C3N4, and BiVO4/PDA/g-C3N4 films coated on fluorine-doped 5

tin-oxide (FTO). The Brunauer-Emmett-Teller (BET) specific surface areas were determined from nitrogen adsorption at 77 K on a Micromeritics Gemini-2390 surface area analyzer. 2.4 Photocatalytic degradation The photocatalytic activity of the as-synthesized photocatalysts was evaluated by adding 0.080g of photocatalyst powder to 500 mL of 0.1 mM glyphosate to form aqueous slurry. Photocatalytic reactions were carried out under visible-light irradiation of a 125 W high-pressure mercury lamp, which the UV light portion (λ < 400 nm) was filtered by a 2 M NaNO2 solution. The high-pressure mercury lamp with NaNO2 solution was immersed in glyphosate reaction solution. The scheme of the photoreaction system and the relative spectral energy distribution of the lamp are shown in Fig. S1. Before irradiation, the aqueous slurry was magnetically stirred and bubbled with oxygen in darkness for 30 min to reach the adsorption–desorption equilibrium between the photocatalyst and glyphosate. Samples were collected every 30 min followed by centrifuging to separate the photocatalyst particles. The absorbance of the supernatant was recorded at characteristic band 710 nm using a spectrophotometer to analyze the final glyphosate oxidation product PO43 concentration through the Mo-Sb-Ascorbic acid colorimetry [23, 24]. The glyphosate degradation percentage η was calculated according to the equation 1:  

Ct

 100%

(1)

C0

Where C0 is the glyphosate concentration in the reaction solution before irradiation and Ct is the PO43 concentration at the photocatalytic reaction time t.

3. Results and discussion 3.1 Composition analysis The phase analysis of the as-prepared photocatalysts was performed using XRD and is shown in Fig. 1. For pure BiVO4, the diffraction peaks correspond well to the pure monoclinic phase 6

BiVO4 (JCPDS Card No. 14-0688) [25]. For pure g-C3N4, the XRD peaks are indexed as per JCPDS Card No. 87−1526; the strongest diffraction peak at 27.9

o

corresponds to the (002)

plane, while the peak at 13.1o corresponds to the (100) plane of g-C3N4 [11, 26]. For BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4, it can be observed that the characteristic peak at 27.9o for g-C3N4 and diffraction peaks corresponding to the monoclinic phase BiVO4 are present, indicating that the crystal phases of g-C3N4 and BiVO4 did not change [27, 28]. It suggests that the BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 composite nanostructures formed. In addition, this observation also indicates that the physical integrity of the BiVO4 and g-C3N4 was maintained when BiVO4/g-C3N4 or BiVO4/PDA/g-C3N4 composites were synthesized. The surface chemical composition and chemical environment of different elements of the as-prepared samples were characterized by X-ray photoelectron spectroscopy (XPS). Fig. 2 shows the XPS spectra of BiVO4, BiVO4/PDA/g-C3N4 and BiVO4/g-C3N4. In the survey XPS spectrum (Fig. 2a) of BiVO4 we can see peaks corresponding to Bi4f (158.9 eV), O1s (529.5 eV), V2p (516.4 eV) and C 1s (284.8 eV). The C1s peak at 284.8 eV is due to adventitious carbon adsorbed to the surface. Similarly, in the spectra of BiVO4/PDA/g-C3N4 and BiVO4/g-C3N4 we can see peaks corresponding to Bi4f, O1s, V2p, N 1s and C 1s (Fig. 2a). The N 1s peak of BiVO4/g-C3N4 could be deconvoluted into two peaks (Fig. 2b). The higher peak at 398.8 eV can be designated as sp2 hybridized aromatic N bonded to carbon atoms (C=N−C), and the other peak at 400.1 eV is attributed to the tertiary N bonded to carbon atoms in the form of N−C3 [29, 30]. However, the deconvolution of the broad asymmetric N 1s feature of BiVO4/PDA/g-C3N4 shows three peaks centered with binding energies at 398.5, 399.2 and 400.7 eV, respectively (Fig. 2c), which the binding energies of the first two peaks are smaller than those of the peaks of BiVO4/g-C3N4. Another peak at 400.7 eV is attributed to the amino groups (C−N−H2) [29, 30]. As shown in Fig. 2d, the deconvolution of the broad asymmetric C 1s feature of 7

BiVO4/g-C3N4 shows three peaks, specifically, a relatively high-intensity peak at 284.7 eV, which can be designated as −C−C; a peak at 285.8 eV, which corresponds to −C=N, and a peak at 288.6 eV, which is assigned to N−C3 [31−33]. As shown in Fig. 2e, the deconvoluted peaks of the C 1s feature of BiVO4/PDA/g-C3N4 can be designated as −C−C (284.5 eV), C−O and −C=N (285.5), and N−C3 (288.1 eV), which show smaller binding energies than those of BiVO4/g-C3N4. Compare to BiVO4/g-C3N4, both the deconvoluted peaks of N 1s and C 1s of the BiVO4/PDA/g-C3N4 shift toward lower binding energies, indicating an interaction among BiVO4, PDA and g-C3N4 rather than the physical mixture. These results suggest that the incorporation of BiVO4 on the surface of g-C3N4 may be carried out through the functional groups of PDA, which is deposited on the g-C3N4. 3.2 Morphology and structure The physical features of the as-prepared samples were observed by SEM and TEM. Fig. S2 shows the SEM images of the BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 samples. As shown in Fig. S2a, the pristine BiVO4 particles show an average particle size of about 150 nm. The pure g-C3N4 sample exhibits sheet structure with wrinkles and smooth surface (as shown in Fig. S2b). Compared to BiVO4, the SEM image of BiVO4/g-C3N4 indicates that the BiVO4 particles are incorporated into the g-C3N4 matrix (as shown in Fig. S2c). Compared BiVO4/PDA/g-C3N4 (Fig. S2d) with BiVO4/g-C3N4 (Fig. S2c), the BiVO4 particles are embedded in the PDA/g-C3N4 sheet substrate. To further understand the microstructure of the BiVO4/PDA/g-C3N4 sample, the TEM analysis of the BiVO4 and BiVO4/PDA/g-C3N4 composite was performed. Fig. 3 shows the TEM images of BiVO4 and BiVO4/PDA/g-C3N4. As confirmed by TEM, it can be observed that these BiVO4 particles are wrapped by the PDA/g-C3N4 sheets. Thus, in conjunction with SEM, TEM aids in further understanding the BiVO4/PDA/g-C3N4 composite nanostructure formation. Therefore, the process involved in the formation of the BiVO4/PDA/g-C3N4 structure is 8

summarized in Fig. 4. 3.3 Optical properties The optical properties of the as-prepared samples were investigated by UV-vis diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra, and the results are shown in Fig. 5. Compared to g-C3N4, the red-shift of the absorption edge of BiVO4/g-C3N4 is observed, while compared BiVO4/g-C3N4 with BiVO4, the absorbance onset almost remains the same (Fig. 5a). As compared with BiVO4 and BiVO4/g-C3N4 the light absorption in visible region of BiVO4/PDA/g-C3N4 is largely enhanced due to the change of the sample color from yellow to green after adding PDA [34]. This suggests that the number of photons absorbed by the BiVO4/PDA/g-C3N4 particles increases. The band gaps are estimated from the plots of (hvα)2 versus the energy of absorbed light (hν) as shown in Fig. 5b [5, 35]; h, v, and α represent the Planck's constant, frequency and absorption coefficient of the material, respectively. Thus, the band gaps of BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 are determined to be 2.44, 2.79, 2.44 and 2.35 eV, respectively. Photoluminescence (PL) spectra are used to observe the electron−hole recombination in the composite photocatalysts, because PL emission is known as mainly originating from the recombination of photogenerated holes and electrons. Generally, the lower PL intensity indicates the lower recombination rate of photogenerated charge carriers, resulting in the higher photocatalytic activity. Fig. 5c shows the PL spectra of BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 using an excitation wavelength of 278 nm. It can be seen that the PL intensity of the g-C3N4 sample is highest among all the samples, indicating that the photogenerated holes and electrons recombine easily and dissipate the input energy as heat. Compared to BiVO4/g-C3N4, the PL intensity of the BiVO4/PDA/g-C3N4 sample decreases, suggesting that there is the efficient separation of photogenerated charge carriers under visible light irradiation, which is the basis for the observed enhancement in the photocatalytic activity 9

of the BiVO4/PDA/g-C3N4 sample. 3.4 Photoelectrochemical property It is known that the generation and transfer of the photogenerated electrons in the photocatalytic process can be indirectly determined by the photocurrent. Generally, the higher photocurrent indicates the higher separation efficiency of the photogenerated electron-hole pairs [36-39]. The photocurrent response measurements of BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 were performed using the film of the as-prepared photocatalysts deposited on the conducting FTO glass slides under visible light irradiation. These results are shown Fig. 6, and it can be seen that all photocurrent responses to on−off cycles of illumination are rapid and reproducible. The pure g-C3N4 exhibits the lowest photocurrent, implying the low quantum efficiency of g-C3N4. Compared to the BiVO4 sample, the BiVO4/g-C3N4 sample shows a higher photocurrent, which is attributed to the higher separation efficiency of the photogenerated holes and electrons occurred in the BiVO4/g-C3N4 heterojunction, thus reducing the photoinduced carrier recombination. Compared BiVO4/PDA/g-C3N4 to BiVO4/g-C3N4, the improved photocurrent of BiVO4/PDA/g-C3N4 indicates the effective suppressing charge recombination at the BiVO4/PDA/g-C3N4 interface under visible light irradiation, consistent with the PL results shown in Fig. 5c. The improved separation of photogenerated charge carriers could lead to the enhanced photocatalytic activity of the BiVO4/PDA/g-C3N4 nanocomposite. 3.5 Photocatalytic activity The photocatalytic activity of BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 was evaluated in photocatalytic oxidation of glyphosate under visible light irradiation. It can be observed from Fig. 7 that a small amount of glyphosate was degraded using pure g-C3N4 as photocatalyst under visible-light irradiation for 180 min. Glyphosate degradation rate was found to be only 55% for pure BiVO4, while BiVO4/g-C3N4 shows higher degradation activity than 10

BiVO4, in which 69% of glyphosate has been degraded. The BiVO4/PDA/g-C3N4 yielded the highest photocatalytic degradation rate among all the as-prepared samples, and 100% of glyphosate was degraded over the BiVO4/PDA/g-C3N4 photocatalyst for 150 min under visible-light irradiation. These results indicate that the photocatalytic activity of BiVO4/PDA/g-C3N4 under visible light irradiation is the highest among all the as-prepared photocatalysts. Compared to the BiVO4/PDA/g-C3N4 composite, BiVO4/g-C3N4 showed a low photocatalytic activity for glyphosate degradation, maybe due to that BiVO4 could not be coupled wholly with g-C3N4 to form the BiVO4/g-C3N4 composite at room temperature. It is known that the specific surface area usually can influence the photocatalytic activity of a photocatalyst. Large specific surface areas always favor good photocatalytic activity. The BET specific surface areas for pure BiVO4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 composites were investigated by nitrogen adsorption-desorption. As expected, the as-prepared BiVO4/PDA/g-C3N4 powder shows a larger specific surface area value (17.7 m2 g1) than those of BiVO4 (9.9 m2 g1) and BiVO4/g-C3N4 (14.2 m2 g1). In the glyphosate degradation experiments, the BiVO4/PDA/g-C3N4 sample with the largest surface area exhibited the best photocatalytic activity. The stability and recyclability of photocatalysts are important for the practical applications. Thus, the stability of the BiVO4/PDA/g-C3N4 sample was investigated by five cycle experiments in the glyphosate degradation under visible light irradiation, and the results are shown in Fig. 8. It can be seen that the photocatalytic degradation rate of glyphosate still remains over 90% after five cycle experiments, as shown in Fig. 8a. In addition, the dark-green color of the samples almost did not change (as shown in the top of Fig. 8a). As compared to the freshly prepared BiVO4/PDA/g-C3N4, in the survey XPS spectrum of BiVO4/PDA/g-C3N4 used after 5 cycles we also can see that Bi, O, V, N and C elements could all be detected as shown in Fig. 8b. In addition, the N 1s spectrum of BiVO4/PDA/g-C3N4 used after 5 cycles (Fig. 8c) is 11

very similar to the N 1s spectrum of the freshly prepared BiVO4/PDA/g-C3N4 (Fig. 2c). Therefore, it indicates that BiVO4/PDA/g-C3N4 exhibits good stability and reusability in the photocatalytic reaction. Based on the results mentioned above, the possible photocatalytic mechanism proposed for the BiVO4/PDA/g-C3N4 sample is illustrated in Fig. 9. The conduction band and valence band of g-C3N4 lie at a more negative potential than those of BiVO4. Under visible light irradiation, both BiVO4 and g-C3N4 can be excited to generate electron−hole pairs. The photoinduced electrons can easily jump from the conduction band (CB) of g-C3N4 sheets to that of BiVO4 or react with adsorbed O2 to yield ·O2− and finally form ·OH [22]; the photogenerated holes react with H2O or OH− to yield ·OH, or hole transfer occurs from the valence band (VB) of BiVO4 nanoparticles to that of g-C3N4 sheets. The simultaneous transfer and reaction of photogenerated electrons and holes in the BiVO4/PDA/g-C3N4 photocatalyst should improve charge separation of photoinduced electron−hole pairs. The PL and photocurrent results also indicate that BiVO4/PDA/g-C3N4 exhibits the high separation efficiency of the photogenerated charge carriers. In addition, the BiVO4/PDA/g-C3N4 sample would generate more electrons and holes under visible light irradiation due to the red-shift in the absorbance and the enhanced light absorption in the visible region, as shown in Fig. 5a. Therefore, the enhanced photocatalytic activity of BiVO4/PDA/g-C3N4 is attributed to the improved visible light absorption, enhanced specific surface area and the increased charge separation.

4. Conclusions BiVO4/PDA/g-C3N4 was successfully synthesized using monoclinic BiVO4 and PDA/g-C3N4 as raw materials through a simple ultrasonic dispersion and self-assembly at room temperature. The BiVO4 particles, which show an average particle size of ~150 nm, were embedded in the PDA/g-C3N4 sheet substrate. Compared to BiVO4 and BiVO4/g-C3N4, the red-shift of the absorption edge of BiVO4/PDA/g-C3N4 is observed, and the light absorption in visible region is 12

largely enhanced. BiVO4/PDA/g-C3N4 has lower recombination rate of the photogenerated charge carriers than BiVO4/g-C3N4, resulting in an enhanced photocatalytic activity. The photocatalytic activity of BiVO4/PDA/g-C3N4 for the degradation of glyphosate is higher than that of BiVO4, g-C3N4 and BiVO4/g-C3N4. BiVO4/PDA/g-C3N4 shows the improved visible light absorption, enhanced specific surface area and the increased charge separation, thus resulting in the enhanced activity of the glyphosate degradation over BiVO4/PDA/g-C3N4 under visible light irradiation. This simple method could be used to synthesize high performance composite photocatalysts for different applications, such as organic pollution degradation, photocatalytic splitting water, etc.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51672089).

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Figures

Intensity(a.u.)

g-C3N4 BiVO4/PDA/g-C3N4 BiVO4/g-C3N4 BiVO4

PDF#14-0688 10

20

30

40

50

60

70

80

o

2 ( )

N 1s BiVO4/g-C3N4 Intensity(a.u.)

BiVO4/PDA/g-C3N4 BiVO4/g-C3N4

N 1s

C-N=C Intensity(a.u.)

O KLL

Bi 4p2/3

(c)

(b) Bi 4p1/2

N 1s Bi 4d V 2p O 1s

C 1s

Bi 5d

Intensity(a.u.)

(a)

Bi 4f

Fig. 1. XRD patterns of BiVO4, BiVO4/g-C3N4, BiVO4/PDA/g-C3N4 and g-C3N4.

N-C3

C-N=C

BiVO4/PDA/g-C3N4 N-C3 C-N-H2

BiVO4

0

200

400

600

800

1000

1200

396

398

400

402

396

404

Binding energy (eV)

Binding energy (eV)

(d)

402

404

(e)

Intensity(a.u.)

Intensity(a.u.)

-C-C

C 1s BiVO4/g-C3N4

280

398 400 Binding energy (eV)

-C=N N-C3

282

284

286

288

290

C 1s

-C-C

BiVO4/PDA/C3N4

C-O&-C=N

280

Binding energy (eV)

282

284 286 Binding energy (eV)

N-C3

288

290

Fig. 2. XPS spectra of the BiVO4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 samples: (a) survey; 18

N 1s spectrum of (b) BiVO4/g-C3N4 and (c) BiVO4/PDA/g-C3N4; C 1s spectrum of (d) BiVO4/g-C3N4 and (e) BiVO4/PDA/g-C3N4.

(a)

(b)

Fig. 3. TEM images of (a) BiVO4 and (b) BiVO4/PDA/g-C3N4.

BiVO4

dopamine

Room temperature, 12 h

Ethanol, room temperature

g-C3N4

BiVO4/PDA/g-C3N4

PDA/g-C3N4

Fig. 4. Schematic diagram of the self-assembly process and growth mechanism of the proposed BiVO4/PDA/g-C3N4 structure. (a)

1.0

g-C3N4

BiVO4

BiVO4/g-C3N4

g-C3N4

BiVO4/PDA/g-C3N4

0.8

2

Absorbance(a.u.)

1.2

1000

(b)

BiVO4

0.6

BiVO4/PDA/g-C3N4

0.2 449.3 nm 400

525.6 nm 511.7 nm 500 600 Wavelength(nm)

BiVO4 g-C3N4 BiVO4/g-C3N4

BiVO4/g-C3N4

0.4

0.0 300

(c)

800 Intensity(a.u.)

1.4

(hva) (a.u.)

1.6

BiVO4/PDA/g-C3N4

600 400 200 0

2.35

700

800

1.5

2.0

2.44 2.5

hv (eV)

2.79 3.0

3.5

400

450

500

550

600

650

700

Wavelength(nm)

Fig. 5. UV–vis diffuse reflectance spectra (a), the plot of (hνα) 2 versus hν (b) and photoluminescence spectra (c) of BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4.

19

-2 Photocurrent Dencity (A cm )

4.5

g-C3N4

4.0

BiVO4

3.5

BiVO4/g-C3N4

3.0

BiVO4/PDA/g-C3N4

2.5 2.0 1.5 1.0 0.5 0.0 0

20

40

60

80 100 120 140 160 180 200 Time(s)

Degradation rate of glyphosate(%)

Fig. 6. Photocurrent response of of BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4. 100

BiVO4

90

g-C3N4

80

BiVO4/g-C3N4

70

BiVO4/PDA/g-C3N4

60 50 40 30 20 10 0 0

30

60

90

120

150

180

Time(min)

Fig. 7. Photocatalytic degradation of glyphosate over BiVO4, g-C3N4, BiVO4/g-C3N4 and BiVO4/PDA/g-C3N4 under visible light irradiation.

40

O KLL

Bi 4p2/3

Bi 4p1/2

N 1s Bi 4d V 2p

(c)

BiVO4/PDA/g-C3N4 used after 5 cycles

BiVO4/PDA/g-C3N4

C-N=C

N 1s BiVO4/PDA/g-C3N4 Intensity(a.u.)

Bi 5d

60

C 1s

Bi 4f

(b)

80

Intensity(a.u.)

Percentage of degradation(%)

(a)

O 1s

the color of the samples

100

after 5 cycles

N-C3 C-N-H

20

0 0

150

300

450

Time(min)

600

750

0

200

400

600

800

1000

Binding energy (eV)

396

398 400 Binding energy (eV)

402

404

Fig. 8. (a) Cycling runs for the photocatalytic degradation of glyphosate over BiVO4/PDA/g-C3N4 under visible light irradiation; (b) the survey XPS spectra and (c) N 1s spectrum of BiVO4/PDA/g-C3N4.

20

Fig. 9. Schematic illustration of the photogengrated charge transfer over BiVO4/PDA/g-C3N4 under visible light irradiation.

21